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宇(yu)宙(zhou)中(zhong)已知的第四大(da)星(xing)(xing)(xing)系(xi)(xi)(xi)(xi)是(shi)ESO 306-17，直(zhi)徑(jing)約為(wei)100萬(wan)光(guang)年。就(jiu)像(xiang)之(zhi)前討論的星(xing)(xing)(xing)系(xi)(xi)(xi)(xi)一樣(yang)，ESO 306-17是(shi)一個(ge)超(chao)巨(ju)型橢圓星(xing)(xing)(xing)系(xi)(xi)(xi)(xi)，但(dan)(dan)有一些東西使這個(ge)星(xing)(xing)(xing)系(xi)(xi)(xi)(xi)與眾(zhong)不(bu)同。當大(da)多數星(xing)(xing)(xing)系(xi)(xi)(xi)(xi)發現自(zi)己處于(yu)巨(ju)大(da)的星(xing)(xing)(xing)系(xi)(xi)(xi)(xi)團中(zhong)時，ESO 306-17是(shi)孤獨(du)的，周圍的空間(jian)都是(shi)空的。天(tian)文(wen)學家認(ren)為(wei)，ESO 306-17可能曾經(jing)是(shi)該地區眾(zhong)多星(xing)(xing)(xing)系(xi)(xi)(xi)(xi)中(zhong)的一個(ge)，但(dan)(dan)它(ta)最(zui)終與附近的每個(ge)星(xing)(xing)(xing)系(xi)(xi)(xi)(xi)合并(bing)(bing)并(bing)(bing)吸(xi)收，導致它(ta)的大(da)小不(bu)斷擴(kuo)大(da)，成(cheng)為(wei)宇(yu)宙(zhou)中(zhong)已知最(zui)大(da)的星(xing)(xing)(xing)系(xi)(xi)(xi)(xi)之(zhi)一。ESo 306-17距離我們大(da)約5億光(guang)年。